It is already known to provide reliable storage and retrieval of large volumes of digital data, such as computer data, in particular by means of the DDS (Digital Data Storage) format defined in ISO/IEC Standard 10777:1991 E.
In the DDS system an elongate recording media comprising tape coated with a magnetic medium is moved by a motor-driven capstan in a DDS read/write mechanism along a path extending between two spools or reels and wrapped partially around a transducer contained in the mechanism and comprising a rotating drum carrying one or more electromagnetic heads. The plane of rotation of the drum is disposed at an angle to the plane of movement of the tape, so that each head traverses the tape along successive tracks extending across the width of the tape at an angle to its centreline.
Each track comprises seven portions or areas, extending in succession from adjacent one edge of the tape to adjacent the other edge: a marginal area, a sub area, an ATF (automatic track following) area, a main area, a second ATF area, a second sub area, and a second marginal area. The data to be stored by the system (e.g. computer data), hereinafter referred to as `main data`, are stored in the main area, together with some auxiliary information (`sub codes`) relating to the operation of the data storage system itself. Additional auxiliary information, relating for example to the logical structure of the main data, is stored in the sub areas.
A section of tape containing a body of main data is itself considered to comprise several successive areas, including a device area, a reference area, a system area, a data area and an EOD (end of data) area. These different areas are distinguished from one another by respective values of an `Area ID` sub code. This Area ID sub code is stored, according to the above-mentioned standard, in the sub areas adjacent the ends of each successive track. Typically an EOD area is marked on the tape after a write session has been completed, and just before the tape is ejected from the DDS read/write mechanism.
One use of the Area ID sub code indicating the EOD area (the `EOD sub code`), and of sub codes relating to the logical structure of the main data, is for fast searching for data stored on the tape. Thus the tape may be advanced past the rotary head drum at a speed several times faster than that used during normal reading and writing of data. The drum heads read the sub codes in the sub areas, and by detecting EOD and other sub codes can quickly locate specific items of data, or the end-of-data, without having to read the data themselves.
However, this fast search technique is completely reliant on successful detection of the relevant sub codes, and in particular the EOD area. If the EOD area is missed the system may try to retrieve data from an area of tape containing old data the first part of which has been overwritten by more recent data, with the result that invalid data may be supplied by the system. If sub codes relating to the logical structure of the data are not detected, the tape may be advanced well past the required location. This in turn requires rewinding of the tape and repetition of the search, thereby defeating the object of a `fast` search. In order to avoid these problems it has been found necessary in practice to limit the speed of advance of the tape during fast search to a value considerably below the maximum possible value at which the tape can be advanced while sub codes are still, in principle, detectable by the rotary drum heads. This is turn has placed an undesirable restriction on the maximum speed at which fast searching may be accomplished.
It is an object of this invention to alleviate this problem.